Agricultural equipment, such as combines, mowers, and windrowers, commonly employ a sickle bar knife to cut the crop. To operate, the sickle bar requires reciprocating motion. Since power on such machines is generally available only in the form of a rotating shaft, some means has been necessary to convert this rotary motion to reciprocating motion. There are two methods in general use which satisfy this need. Both involve purely mechanical construction. In one method, the rotary motion is converted directly to reciprocating motion by means of the simple crank and connecting rod mechanism. The other, more sophisticated method, as exemplified by U.S. Pat. No. 2,619,839, employs a wobble shaft, which, when rotated, causes a yoke to oscillate by virtue of its connection thereto by a set of mechanical bearings in a gimbaled arrangement. Reciprocating motion is then obtained by mounting an arm on the output shaft of the oscillating yoke. Both of these methods are subject to major complaints. The life of the mechanical anti-friction bearings is short because of the nature of the oscillating loads and the inevitable presence of backlash and shock loading as a result thereof. Mechanical drives have limited speed capability. This complaint is related to the previous in that, to achieve higher speeds would cause even higher magnitudes of shock loading on the bearings, further shortening their lives. Higher sickle bar speeds are desirable in order to allow higher vehicle ground speed, thereby increasing the machines productivity. The transmission of the vibrations caused by the shock loading into the framework of the machine is also objectionable in mechanical drives.